Like I'd said earlier, this is a judgement call. If there are inexpensive replacement phones available that are better than what he has now then there's nothing wrong with buying one.

A shrewd move may be that he could do would be to get a newer phone, try to install Lineage OS on the older phone and, if successful, keep it as a backup. Depending on how happy he may be with that, he could forgo paying for any insurance for the newer phone on his monthly plan. (I don't know how common that may still be - I keep a backup phone)