I have seen this scenario recently.
There are many golden relationships right now where people live apart or together but do not get married or sign a legal agreement. Many of the seniors have children or grandchildren from previous relationships. The couple may spend much time together and at times purchase items, trips, services for each other. At times one will put something on a credit card and get paid by check by the other person.
When serious illness, death or dementia occurs a family member can scrutinize the financial records and make accusations of financial abuse and it can get NASTY.
So if Mary is putting a sofa on a credit card for John and John is giving Mary a check for the purchase, make sure it is well documented. If not Mary can be accused of taking money from John when in reality she was not.
Legal advice can guide you to protect yourself and the person you care about. Yes, it costs but it is worth it.