If his future fiancée were opposed to such a tradition he would know about it.
Some women can find offensive.
However if he thinks she'd be fine with the gesture then I'd go ahead with it.
But frame it as pp said.
The FIL can't say no.
Harry and Meghan ‘royal’ tour of Nigeria.
The Republic of Ireland and their tensions with migrants.
Forgetting where you left your keys does not mean you are “losing it”