Well, I would imagine that your daughter is probably not a virgin any longer. No 20 year old guy is going to refrain from sex with his girlfriend. So, first and foremost, get that girl on birth control.
Now, she will most likely deny having sex with him. But, insist on birth control if you do not want to be raising her child.
You can very well forbid her from seeing him, but that may just magnetize her to him all the more. Even still, a 20 year old dude is way tooooo mature than a 15 year old naive girl. At their age, there is a HUGE maturity difference and as a parent myself, I would be very concerned.
A 15 year old girl who is "in love" has zero reasoning skills, so expect some backlash, lies and drama to follow if you forbid her from seeing him. Personally, my spouse would threaten to skin him if he ever had contact with our underage daughter. But, the question is: Can your daughter stay away from him?