Mind her own...business...

Your colleague would not be in a position to authorize payment for you if you did extra hours so is she expecting you to help-out for nothing? I experienced a lot of jealousy at work because of my part time hours but I would then reply that they wouldn't want my part time money!

Why did she ask you, of all the others. If she works "nearly" full time, she's got time to work... so why doesn't she do it herself? You went in to that position with a job description. i.e. Fridays are your off days. You are not obliged to take up her suggestion. She wants to look good with Management because she'd have "found someone" to do the job. She didn't even mention tat your paycheck would be increased.

Just say it's not possible. There's no point undertaking additional duties if you are going to resent them.

Why has she taken it upon herself to ask you to do extra hours when it’s nothing to do with her, it’s up to your employers to either approach you themselves or replace whoever has left.
I would just say to her you don’t want to do it and leave it at that.

Please tell your colleague that with respect your hours and conditions of employment are none of their business.
Simple as that, no more discussion or conversation to be had.

I worked in a busy office, and one colleague left abruptly - to put it mildly. She just upped and stormed out. We were in a fix, and I was asked to help out, which I willingly did for quite a while, putting myself under stress as a family member was in hospital. A kind colleague took me to one side and said that I had done my share, and it wasn't MY responsiblilty to cover for absences in the office. It was a light bulb moment - I went to see the manager and said was he making any effort to get a replacement? No, he wasn't! I said that I wasn't going to continue with the impossible workload further than the end of the week - and hey presto, a replacement arrived from a temp agency on Thursday. The point I am trying to make here is that we perhaps all feel a duty, a 'Protestant work ethic' but in truth it is not up to you to cover for colleagues more than if someone goes home ill and is likely to be back in a couple of days.

Stick to your guns pet

Hope the responses you have received have helped you. GrannyorNanny, they mostly seem to be singing from the same hymn sheet. When I was working part time (many moons ago ☺️)
I used to get into the lift at 1pm to go home and one of my colleagues would be there going for her lunch every day without fail she would say to me must be lovely to work partime and be going home, and I would reply everyday Oh it is you should try it .
I think some workers not all by any means do feel a bit of resentment to P/T workers.
Also remember the old saying no good deed goes unpunished.

Perhaps your colleague was just trying to be helpful. Maybe she thought that you would welcome the extra day’s work and extra pay. As you are happy with the status quo just let it lie, unless your manager approaches you with the offer. Then you can politely refuse. You don’t need to give a reason, just a ‘thank you for offering it to me.’

I find it very hard to no!
Before I got ill someone left without working notice.
I was asked would I cover until they got someone else
Yes I said as long as it isn’t too long
3 months later I was still doing the extra hours!
7 days a week 8.30-5.30...
When I mentioned it several times I was told they haven’t found anyone suitable
One day I completely lost my temper and said if it isn’t sorted by the end of the week,your be looking for 2 new employees
So stick to your guns and don’t be a mug like me

YANBU Take it you are not responsible for the staffing in the place. Who ever is should be addressing the shortfall in the other team certainly not you.

I wonder if your colleague who works "nearly full time" is concerned that she may be asked to up her hours and now go full time to cover the work - possibly doesn't like the thought either and therefore is suggesting you work the Fridays. On the other hand may have no ulterior motive at all. I would wait until management decide what they are going to do about it, and if you are asked if you are interested say no if you don't want it.

If you don't want to do it do'nt.
If you do it once you will be stuck with it, as everyone will assume you can always provide cover.
You would be making a rod for your own back.

GrannyOrNanny, in regard to your query, You are obliged to work the hours set out in your employment contract terms. Your contract may also state something specific in regard to overtime, For example, very many employment contracts state that "a reasonable amount of overtime may from time to time be required, in accordance with the needs of the business".

The above terms are very much included in employment contracts drawn up in the Transport Industry, retail services, parcel home delivery or any business where customer demand can be volatile for any number of reasons.

The best practice for any employee is always to discuss such matters with their direct line manager. As you state GrannyOrNanny in your opening post that the situation has only come about due to the loss of an employee on a different team, then it may well be that a temporary solution is required by your employer while a new employee is recruited.

Reference to overtime working can be found by following this link:-
worksmart.org.uk/work-rights/pay-and-contracts/hours-work/my-employer-wants-me-work-extra-hours-excess-those-set-out