There is more than one issue here
1. The vagina is accomidating. It stretches and tightens to accomidate various things.
2. All women are different. Some are larger than other.
3. Keigel exercises are proven to work, moderately. Research it.
4. One factor coube be wetness. An overly wet inside feels looser, less friction. Use a towel or something to wipe his penis and the inside of the vagina. Make sure to leave a little bit of moisture so as not to rip upon re-entry.
5. He is not an ass if he said it right. If he said: "damn, you are so loose." that is bad and she should kick him in the nuts. If he said: "honey, I have been noticing that you have been feeling a little loose. Have you noticed a change in the sensation of my penis?" that is a good way to ask!