I don't know anything about Scottish law but if she is using an agent for rent collection and/or management, then I'd put pressure on them to start sorting it out. Presumably they have experience with this sort of thing. Point out that if it does come to eviction, shewill be asking for a pro-rata proportion of their fee back and won't be using them to find new tenants.
AFAIK, once he's three months in arrears, eviction proceedings can be started, but if less than that amount is outstanding once it gets to court the eviction won't be granted. (Ie, if he's managed to pay off some arrears.)