If your friend is well off, the lawyer knows it. He is paying her to defend him. If he could file a countersuit at this time and profit was her motive, logically, two jobs pay more than one, especially if he stands to collect from the countersuit and she can get her mitts on a percentage. For this reason, I doubt greed is the reason she told him no.
Idaho has some very strange laws. That is why so many oddball Supremacist and anti-government groups are centered there. It would not surprise me in the least that they have some quirky law against simultaneous countersuits. Most states allow for counter suits and most lawyers encourage them. If your friend is suspicious, he should consult with another attorney. Second opinions...they're not just for doctors anymore.

Yes. It's called counter-suing.

Although your questions have been answered well, I took some neighbours to court for harassing me. The lies they told were astonishing, they went to great lengths to make me the guilty party, when I had taken all of their harassment, even the total destruction of my gardens - which they said in their signed Statements, I did myself! I was in the US at the time!

The stupid Judge in all his wisdom, actively encouraged all 5 of the defendants to make a counterclaim, saying they still had time to do it (meaning after lunch!), incidently, they never did! I don't know whether, by that, he meant it would be dealt with there and then but I suggest your friend phones the court, they should be able to tell him about the procedure regarding counterclaims.

The Law is an Ass and will always be an Ass! The whole thing was a complete Charade, I could write a book about it! My eyes were opened wide. All I gained was the experience of how farcical it all is, I took a 4 day trial myself - only for the Judge to throw the case out at the end of it! I learned afterwards, that only one Judge insisted on reading all of the evidence before he took a case.
The first thing the Judge said when he opened the trial, was that he didn't know what the case was about and had only just been given the files at 10.25, we stood before him at 10.30. He then went on to blame the Government for shortage of staff! My request for an Adjourment on those grounds, was refused!

No, he meant them to go and do it in their lunch break.

counter suits are permitted in Idaho, see if the court has a small claims adviser and talk. check with the bar there for $25.00 here you can get a half hour.