^asalam alaikum, you could ask them to bring evidence.
other than that, i've not heard that this situation makes a nikah invalid. Sometimes a nikah takes place (can't remember the term used) but for some reason the couple cannot live together, e.g. ppl still who are still studying. They continue to live with own parents until they are ready for the responsibilities of marriage. That can be until any length of time and it certainly does not invalidate a nikah. Allahu alim.
The only thing that comes to mind is when a marriage has not been consumated and divorce follows, there is no idah period to be observed. (Can provide proof if you like). But again no mention of the three month period anywhere.