[manuelflara]

Just because your co-founder just now feels like "working on it alone" doesn't give him/her the right to force you out, specially if he or she doesn't have any leverage (as you say, the investor is remaining neutral). So what is preventing you from just saying "no, thanks, I'll keep my 40% and keep working on this". What would he/she do, then?

[tartoran]

What keeps the OP from saying they'd rather work alone on it and the other cofounder leave instead, then see what the co-founder want to leave. If they come with some amazing demands, the same demands can be used by the OP to leave.