Backblaze gets very good reviews - it is strictly a backup service, no file sharing, no sync service, just backup and restore (and only data files, not the OS or any applications). $50/year, unlimited storage.
Personally, I just don't trust any online service - you are then putting your data in someone else's care and trusting it will all work out if needed. I use hard drives - one for a Time Machine backup (about as simple as you can get), and two for whole system clones (one kept off site).
Honestly, you need to convince your friend that if these data are truly precious to her, then she needs to "suck it up" as they say and learn how to implement some good data backup and recovery practices. Ultimately, nobody looks out for you and your stuff other than you.
P.S. even my 86 year old Mom uses Time Machine and a hard drive to backup her iMac - TM is really about as painless and simply a system as you could ask for.