Hi Rock,
A good shareholders' agreement would contain an "offer round" procedure for buying each other out. In other words, if one person wishes to get out, they would offer to buy the other for a price they set. If that person does not wish to sell, then they must buy out the offering shareholder at the price offered.
Do you know what I mean?
This is triggered usually only in particular circumstances.
Both director-shareholders should also have Directors' Service Agreements - ie. employment contracts for directors. These contracts would require them to behave in certain ways. If one of them is not complying with their contract the contract can be terminated, which again might trigger a requirement to sell the shares back.
All of this documentation should be put in place when a company like this is set up. Unfortunately, it often is not, as people are reluctant to pay the relatively tiny legal fees.
Your friend needs to go to see his solicitor and discuss the matter. There are very complex provisions governing director's duties, and shareholder rights and responsibilities set out in legistlation, even if the proper contracts weren't set up in the beginning.
There is a lot your friend can do, but I can't advise on the limited information available on a public forum.
Your friend should see a good solicitor with experience in Company Law.
Good luck,
Kate.