LuckyStar, a good agent should certainly have gone on trying beyond a couple of rejectsion, unless the feedback they got was so negative that they themselves have lost faith in it. Giving it to a colleague to judge is a good idea, but you should have heard something by now. Agents don't like giving bad news - by definition they're bouncy, enthusiastic types, or should be - and are always liable to just not get round to telling a writer that it's not working. They also tend to be busy, not natural administrators, and have to spend most of their time on the clients who are paying the rent.
It seems to me that your friend's got various options, if they're not answering phone calls and emails: she/he could
1) Write to the agent friendlily asking what their plans are for his/her work now, and saying that she/he would be grateful to know something of any feedback they had from colleagues or the publishers concerned. (But brace yourselves, as it may not be cheerful reading).
2) Write to the agent (politely - it's a small trade) saying that it's obviously not a novel they can find a home for, and that your friend would like it back, in order to try elsewhere. Say that if she/he hasn't heard within - say - two weeks, she/he'll feel free to offer it elsewhere. Not much chance of friendly helpful feedback after a letter like this, but you're within your rights to ask who they sent it to, as any new agent absolutely has to know this.
These are the ethical options, since a new agent won't want to be seen poaching off the old one. But they do include facing up to no longer having an agent for a while. I've been there, and it's not nice after the thrill of having got one.
3)Less ethical - but given how this agent is behaving, I can't say many would blame your friend - is simply to send it out again, wait till there's a bite, then ditch the old agent briskly. But I'd keep quiet to the new agent about the old agent until you know they're interested - at the first meeting, if not before. Your friend must tell them about where it's already been fairly soon, though. An agent reads a possible new MS already thinking hard about which editor will like it, which imprint, who's got money to spend, who's not got anything quite like it, etc... When the new agent realises you've been doing this, it might not be the absolutely best start.
A last thought. Does your friend have a contract or letter of engagement with the old agent? If not, asking for one could just propel them into action. If she/he does, a copy of that and a cheque for £85 or so will get him/her membership of the Society of Authors, and therefore unlimited free advice clause-by-clause on contracts, agents, editors, tax, copyright and everything else thereafter. Including how to handle this situation. They also have very good leaflets on issues like this, free to members, a few pounds to everyone else:
http://www.societyofauthors.net/index.php4
I hope your friend goes on knowing that having got that far - having had an agent that thrilled to start with at least - is in itself an achievement, and not one that's diminished by things not having got further this time! Good luck to them.
Emma
<Added>Final thought. Is your friend's agent/agency a member of the Association of Authors' Agents? Lots of perfectly respectable agencies aren't, but the AAA does have a code of conduct about how their members should behave, which you can read on their website - sorry, don't have the link. Even if yours isn't, a clear knowledge of how a good agent should behave should strengthen your friend's case with this one.