It’s a non issue in this case.
But for clarification purposes. Joint accounts are technically and legally 100 percent the property of both account holders.
If you are on the account you can walk in and empty it if you choose. There are other account options that can differentiate ownership. JWROS is the traditional joint account. It’s not just 100 percent yours in life, the right to these funds are also 100 percent yours if the other joint tenant passes away.
In fact being added to a joint account for non spouses is a reportable gift above the annual exclusion amounts. The only reprieve is to deem it to be an account of convenience, in the example of the adult child on there to help a parent. However when that parent dies it’s funny how many times the account of convenience concept goes out the window and siblings get into a battle over the money. It’s really hard to get this reversed. Caveat emptor.