How do I tell friend I don’t want to travel with her?

I don't think that travelling in South America is advisable if this lady has serious health issues .
It's your dream trip and if you have to worry about her - it really could turn into a nightmare .
I have several friends with whom I wouldnt consider going on a weekend away with let alone South America.
If it came to it -I'd have to say no .
Before she makes lots of plans to come with you - nip it in the bud by telling her that it would too much for her .
One of my friends ruined a trip to North Africa .
I had two weeks of her neuroses and dilemmas and I felt frazzled and fed up with her.

a paint fight in my bathroom (Grammaretto)

The mind boggles! 🫟

Friends and I have 1/2 night breaks away and I’ve always insisted on a room each even for that short time! The only person I’ve ever shared a room with is my husband and that has had its challenges on occasion…

I don't even share a room with OH! He has snoring issues and needs the loo umpteenth times in the night. I've shared with my daughter a couple of times but she grinds her teeth and I snore a little. I think on our next trip we will have separate rooms.

nanna8

Tell her you snore loudly and book a single room!

I do snore loudly and worry so much about it that I hardly sleep and am like a zombie the next day when I have shared a room!
Single room is best.

MT62

Tell her you have chronic sleep issues, and that on second thoughts you would much rather sleep on your own in your own room.
It’s hard enough sharing a room with a good friend, never mind someone you hardly know.

Exactly. No way would I share a room with anyone but immediate family. I’d tell her I don’t sleep well and am up and down half the night, putting the light on and making cups of tea. (True for me anyway!)

Pleasebenice - I have been to visit my brother and family in South America - they were there on a work exchange.
You are brave to travel alone or are you on an organised tour?
Regardless, you will enjoy it - such a beautiful country, but scary too. Everyone carries guns ….

Just
Say
No.

or if you need to pad it out
No. That doesn't work for me.

Do not say sorry.
Do not parly.
Stick to your script.
Repeat as necessary.