My long-term girlfriend's best friend (married) often hosts her for sleepovers
When she travels to her college town nearby.
They have a fun time and they catch up on life etc.
Over the past week her friend's husband indicated an interest in my girlfriend
via text, and she said to stop, and he called her and apologized. They have all known each other for years. This weekend she is heading to town a day ahead
And said she would like to sleep there, and obviously doesn't care about him.
I expressed some hesitancy and said I may be uncomfortable and she said she
Wanted a free place to stay and not to worry. Should I spring for a hotel? Is she being insensitive? Should I have right to express discomfort?

"Should I spring for a hotel? Is she being insensitive? Should I have right to express discomfort?"

You can spring for a hotel, but she isn't obligated to take it.

Is she being insensitive? Can she ask you if you are trusting her?

You have the right to communicate if that includes expressing your discomfort. But I don't think you have anything to worry about based on what you have posted. She told you about it, so trust her to make the right decisions. If the guy gets out of line your girlfriend will probably tell her friend - and he probably knows that.

Don't make her regret being honest with you.

The $64K question is do you trust her? If you do, then this is all about you and not about her. She can, apparently, handle herself.