Yes, and I find that many young people whose lives end up being in turmoil, such as a separation/divorce.. end up landing at their mom's or parents' places, and many of these parents seem to accept this with open arms.
Again... what I am saying, is that just because that is what you believe, doesn't make it true. When I was in my twenties, NO ONE I knew went to bed early on weeknights, no matter what time we had to get up.
ETA - If the relationship was recently in seperation, then yes... Short term crashing is very likely while they figured out what to do long term. Maybe they were like many twenty year olds in a relationship, and were apart then together, then apart again, then with someone else, and back home again. Many, many variables.