Hi kia. Hope you and your daughter are doing ok.
Speaking as a mum I know how sh!tty it can be when you know that someone isn't right for your daughter but there's nothing you can do about it, knowing it's going to end in tears, and it's going to be theirs.
Without trying to be patronising, being only 19 and having been in a two year relationship does not leave much room for becoming 'worldly wise' as it were and I think that dealing with a partner's pansexuality at her age is probably a bridge too far. She loves him and is desperate to hang onto that love but chances are she's going to get hurt and he's going to use his sexuality, pan or otherwise, to justify his behaviour.
Is it possible that you or your partner (if applicable) could talk to him about what's going on? Perhaps his 'branching out' with hair, make up etc is a sign that HE needs to move on. Maybe he could be the one to take responsibility for ending things so that he can go and experiment/fulfil his sexual needs without leaving your daughter hurt in the process. It'll be distressing for her in the beginning but long term it's probably the better option.
Just my tuppence worth, please feel free to ignore :-)
Good luck.