I would ask my friends why they feel it is completely inappropriate if they said that since they know y’all well and you can talk that out.

13 is very young and has little in common with a couple in their mid 20s. Least on an emotional intelligence level to a degree as well as life experience and most likely interests and the like.

You said you think the dad sounds like an alright enough father. Is that from what you have gathered from the girl? Do you two both know him on at least an friendly acquaintance/neighbor way?

Is he aware his daughter is at your house watching movies and spending time together at your home at least once a week? What about the Mom, is she out of the picture completely?

It would also be helpful to know how long you all have been close to this girl for and if there has ever been any other friends that come by, maybe a boyfriend.

If anything this is a good topic to crowd source to gauge what out side perceptions are and to help with self awareness in the interest of what is best for everyone.