If you or they are not technically inclined, write them down on a piece of paper, stored safely.
If you are, encrypt a file or volume on your computer and use that.
I've done and advised this forever and each little story like this leaves me convinced that these ways, while not perfect, definitely beat all the others.
Right, I suppose what I mean is "local software" over "centralized service."
Frankly, I'd even avoid Dropbox here. No need; and slightly reduced threat model (e.g. you happen to pick a bad encryption scheme). Syncthing, if anything.
If you or they are not technically inclined, write them down on a piece of paper, stored safely.
If you are, encrypt a file or volume on your computer and use that.
I've done and advised this forever and each little story like this leaves me convinced that these ways, while not perfect, definitely beat all the others.