Hi @YvonneMC - congratulations on your dear grandson, glad everything is going ok now.
Do you mean like a breathing monitor mat, that goes under the mattress when baby is sleeping?
e.g angelcare.co.uk/product-category/monitoring-filtered
If so, then yes, I personally definitely would suggest you buy them one - regardless of whether your darling grandson was premature or not. Whilst not compulsory, they do give parents peace of mind that the baby is breathing. Yes, they can alarm falsely (babies sometimes hold their breath slighly longer than normal) but if there are any issues, the parents know about it straight away and can action.
Sorry, if you mean something more medical, i am not aware of everything on the market.
If you do mean the monitor mat, by DD has the angelcare one with sound sensor. I think they paid about £80 for it 3 years ago on sale. They say that the monitor along with babyborjn bouncer and their swivel car seats are the best baby investments they made.
I don't think you are interfering at all, i think it is a very generous, thoughtful offer.