Does "get together with him" mean have a date or have sex in this context? I'm guessing sex?
I was uneasy with it, and told her as much, but told her she was free to do what she wanted, not wanting to restrict her freedom.
Could it have been more honest/accurate for you to say "That guy? I'm not crazy about it. He's not been honest and he is irresponsible so I would just prefer you date someone else other than him who is more trustworthy. I'd worry about your well being with him."
If so, why not just state your preference from the start? You are partner to your GF. She's responsible to herself, of course. But you are also obligated to help watch out for her emotional health, mental health, physical health, and spiritual health.
You could be giving full clear information about how you feel and information about him that you know -- he's not an honest guy. Be aware, and be careful of your emotional and mental health, GF!
-- so she could make her own choices from a place of full information.
And if this person is irresponsible and dishonest, why do you remain
friends with him?
Sometimes it's not about "learning to get ok with something" but rather "accepting I am just NOT ok with this and won't ever be!"
Be pickier about who you enter into friendship or polyship with. (You and GF.)