I would recommend hitchhiking to the border crossing by foot and hitchhiking again.
If you and your friend are more comfortable with public transport you can go to Podgorica with the train (that's about a hour) and then take the bus to shkoder which is about 2 hours. So with transfer times maybe 4 hours. Here are the train times from about 2 weeks ago. Not the best picture. Sorry for that.
Train and bus are still a long travel time. So maybe ask at the bus station if there is a bus to the border. I couldn't find any online. Or if you have the money a taxi.
Hope you are enjoying Montenegro I was there about 2 weeks ago, it was very pretty and the people seem to be very friendly.