75 minutes is plenty of time to connect in DEN. All Frontier flights are on the A concourse so you'll have no more than a 5-minute walk from gate to (furthest) gate.

In the highly unlikely event of a missed connection, Frontier will first try to put you on a later flight of theirs; if this is more than a couple of hours later, ask them to interline you on AA or US if they can. Frontier has an interline agreement with AA and US but not with UA, as I remember.

You have the best chance of interlining if you don't have checked baggage.