(Basking in the adoring cheers of the crowd. 
)
Awesome, Arthur. Though I will say that the parens had better have a lot of clout within House Tremere if he wants to actually try to readopt his apprentice, as Tremere is a True Lineage and allowing a "return to the fold" would require a major policy waiver.
Well, if the apprentice lives as a Trmere should, spent the formative years of his apprenticeship under the tutelage of a Tremere, it isn't too hard of a stretch to see it. Also, keep in mind the apprentice could be invited to Transylvania, out of reach of his Bonisagus parens and spurn any attempts to be contacted by him.
Yes, this may be difficult. I would require the apprentice to really want to leave House Bonisagus to become a true Tremere again. And if he's really that minded to do so, I'm sure a period of intense coaching to complete his "true" apprenticeship in Tremere traditions might be able to solve this. He did spend, after all, 9 years in the lineage.
But it is completely optional to the plot. The former apprentice might settle for working closely with/for House Tremere. JL mentions one possibility. Another might be moving to the covenant of his first master, so as to protect future apprentices from further poaching. Calling the Tremere his "true pater" publicly. Voting against the Bonisagus and his covenant at Tribunal. Or publicly surrendering his voting sigil to the Tremere, as is done in the tradition of that House. Things like that.
In short, the Bonisagus will have gained a new enemy. One that knows many of his secrets (and those of the covenant), as he lived and worked for him in the lab for many years. He might actually have acted as a passive spy while still an apprentice. All without using any magic at all.
Which leads to another point. Even if he does use magic to spy upon his new master or his covenant-mates, the scrying clause cannot affect him since he is an apprentice. It can only cause punishment to the Bonisagus, not the Tremere. Who would be very careful not to provide any magical assistance to his former apprentice in such endeavors. The Tremere might even advise his former apprentice not to use magic for this. But collecting items to act as Arcane Connections is not using magic... Nor is gathering information about the Bonisagus' current lab projects, the spells he knows (or doesn't know), the security measures that the covenant uses, sensitive mundanes that work for the covenant (and could be turned against it). This makes the Bonisagus' covenant very vulnerable, should the Tremere and his covenant plan on making trouble...
But if I was the Tremere, I would take the long-term view and wait until the apprentice has Gauntleted before using all that information. That way you keep him safe and come at the Bonisagus from two different angles at the same time.