They have been involved since last Christmas, and she graduates in 18 months.
They should wait until graduation and can support themselves without help from parents.
If they are financally independent, then it’s their decision.
I’d approve of marriage at or after graduation. That gives them time to discuss kids, plans, expectations, and everything else . . . and to see if there are any problems they didn’t notice yet. Sometimes people are on their best behavior at first, but that’s tough to sustain for two years.