Yes, in this very update you can start the beginning of it. You must tell wild Holly that she is hot or fit and have at least 5-6 Relationship points with Holly, but more is better. Still, if you like Holly, the RPs should be no problem, I had them maxed out by the time of the beach trip.
Also if you break up with Lena just at this beachtrip, then you have to make sure that Emma and Perry hit it off, due to the bedroom situation.
By the way "cheater Holly" is a major misnomer! It becomes crystal clear in game that most of Holly´s "slut route" was done by her in the (partly mistaken) belief that Ian prefers wild girls and changed to have a chance with him.
Now that she has Ian, I doubt her drive towards sluttiness will remain.
As I mentioned in some earlier posts, going by what we know currently, there are 3 options how it goes on if they become a dedicated couple:
a) Holly reserves her sluttiness for Ian in their bedroom
b) Ian likes Holly´s wild side and they do some small further experimentation, say erotic pics or her going topless at the beach, until they find a common sweet spot
c) Ian likes Holly´s wild side really much and they experiment a lot further. But even then Holly is no cheater since they do it together, not behind the back