I seem to recall that New Judge has largely answered this on one of your earlier threads. (He really is a judge, so he ought to know what he's talking about!).
As NJ stated, "the court must activate the custodial sentence unless it would be unjust in all the circumstances to do so". (That's not just NJ's wording. It's a direct quote from the document that magistrates and judges MUST refer to:
https://www.sentencingcouncil.org.uk/offences/magistrates-court/item/breach-of-a-suspended-sentence-order/ )
As that link shows, the judge first has to consider whether the new offence merits a custodial sentence. If so, the suspended sentence will be activated but can be reduced if your b/f has been complying with all other aspects of the original sentence (such as doing any unpaid work that might have been ordered or attending probation appointments, etc). If there has been liitle or no such compliance though, the full sentence will be activated.
If the new offence doesn't, in itself, merit a custodial sentence, the suspended sentence can be activated (but reduced for 'compliance', as above) or its term can be extended (and/or additional requirements added to it) or, probably unusually, replaced with a fine.