Her parents are having trouble finding their most recent mortgage statement but managed to find the one from 2015. I'll ask them to keep looking for the one for 2017 just in case, but if they can't find it is this okay?
It's fine - you don't actually need to provide a mortgage statement from them at all, so if you can't find the 2017 one, I wouldn't bother including one at all.
It's not a required document, we just suggest sending it so you can show they are keeping up with their payments (meaning they are unlikely to default on the mortgage and lose the house).
Also, she just showed me her bank statements and she got them printed at the bank. The bank printed them on letterhead (at least I think it is) and the bank employee stamped the first page with a stamp saying "I certify this to be a true copy of the original" signed and dated the statement and put down her bank staff number. Does she need to get each page signed and dated still since it's not the original copy?
If they were printed by the bank themselves, and they show this (i.e. they are on letterhead paper or they state 'printed in branch on X date'), then they don't need to be stamped on every page.
She was also wondering if she could highlight her pay deposit for each month since her statements show all of her transactions?
No, she should leave them completely untouched - UKVI will know what they are looking for on the statements. If she starts highlighting things or blacking things out, that could be seen as tampering with original documents.