That is something to think about! Since many people can’t seem to agree on what a cult is, then I would say that is a very good question. I probably would have asked him what his definition of a cult was so that I could understand his rhetorical question better.
From Merriam-Webster online, this is the definition of a cult (or rather definitions). According to #1, #2, #3 and #5 BOTH the LDS and RCC are/ could be considered cults
Main Entry: cult
Pronunciation: \ˈkəlt
Function: noun
Usage: often attributive
Etymology: French & Latin; French culte, from Latin cultus care, adoration, from colere to cultivate — more at wheel
Date: 1617
1 : formal religious veneration : worship
2 : a system of religious beliefs and ritual; also : its body of adherents
3 : a religion regarded as unorthodox or spurious; also : its body of adherents
4 : a system for the cure of disease based on dogma set forth by its promulgator
5 a : great devotion to a person, idea, object, movement, or work (as a film or book); especially : such devotion regarded as a literary or intellectual fad b : the object of such devotion c : a usually small group of people characterized by such devotion
— cul·tic \ˈkəl-tik\ adjective
— cult·ish -tish\ adjective
— cult·ish·ly -lē\ adverb
— cult·ish·ness -nəs\ noun
— cult·ism \ˈkəl-ˌti-zəm\ noun
— cult·ist \ˈkəl-tist\ noun
— cult·like -ˌlīk\ adjective