Whether this will be easy or difficult will depend on the attitude of the owners, how they own the property and if they are sensible about this. The route your client will need to take will depend on whether or not the four of them bought as joint tenants or as tenants in common.
If they purchased as joint tenants then your client can apply for an Order for Sale to the courts. The proceeds after sale would then be divided equally after selling costs and payment of the mortgage redemption.
If they purchased as tenants in common (more likely as they are friends rather than a married couple) then a Declaration of Trust should have been entered into when the property was purchased. That declaration should have set out the interests of the various parties. If no declaration was signed, then the interests of the parties will be determined by the court and the court will look at their individual contributions. Obviously going to court is not an attractive proposition and that is why the parties should be sensible and see if they can not come to an amicable resolution ‘ but if all else fails then off to court your client can go.