Well, if he alleged to hit one of the residents then they have to suspend him. No choice.
In this case before any disciplinary hearing there will need to be a police investigation potentially first (depending on what they say) as it is of a criminal nature.
If he is not a member of a trade union, even if he signed up today, It will be at least 6 weeks usually before they will even support him formally. They may still not help represent him at any hearing either as this is before he joined them if he does join one.
With any allegation he is entitled to a fair trial by his employers. It therefore depends on what the police say (assuming of course they have contacted them as alot dont, when they should do). They will need to present the evidence to your friend so you can answer the allegation. Your friend will be given a copy of the disciplinary procedure and what to expect.
I would advise he takes someone along for support. The citizens advice bureau I think are rubbish but in this case he should ask for their advice and also goto
http://www.worksmart.org.uk/rights/He should be given a point of contact as well to discuss or ask any questions.