How do you deal with this kind of Groupmate?

I can't think of anything you could do, except for confront them about the situation. Or perhaps you could talk to the other group members and come to an agreement - and maybe confront the one person "intervention" style (?).

Two ways I would go - Confront them on the issue, though they seem headstrong about it. If you're too nice and polite, they'll just keep on with what they're doing, and if you're too assertive, they'll just oppose the confrontation. The other way to go, encourage the groupmates to all take action and delegate things. Increase their participation in making decisions, even when the bossy one tries to dominate the decision-making. If the bossy groupmate is still pushing their own schedule and delegations, don't be afraid to ignore their demands once and a while. You can still consider their input and not go down their path. Overall, find a way to remind them that it's a GROUP project, not a group of people bearing up an individual to glory.

well one way is not work with this person anymore. when the next group project comes along, exclude him from the group. this should send a message to him loud and strong. but while your friend is stuck, he might as well make the most out of it. i guess the leader is striving for good grades thus his bossiness. just rough it out and your friend will be awarded with good grades too. the longest group project i've ever worked on was for six months. so bear with it for this period of time and be done with him.

I guess, if the rest of the members feel violated with the bossiness of the group leader, they can assert to this person that they no longer need his "leadership" and they can negotiate with having to delegate certain roles.

does your school have such things are peer evaluation? Well for me, we do. So we mark them down on peer evaluation. Peer evaluation is like you rate your peers according to percentage. So let's say, the accumulated percentage your group rates that group leader is 20%. This group leader only gets 20% of the total report marks. Of course, there would be a remarks section for you to write your views and reasons. If your school doesn't offer such an option, perhaps you might want to speak to the lecturer about it.