Jim in the comments:
I have this suspicion that Corbyn is very carefully trying to make sure that he has nothing whatsoever to do with Brexit at all, and is leaving the Tories to sink or swim themselves over it. On the wise principle its better to let others make decisions that have only hard outcomes, and let them get blamed for them. Who now remembers that the Tories pretty much unanimously voted for the war in Iraq in 2003? No-one, Blair and Labour get the blame.
So I suspect he will studiously ignore May’s entreaties to get involved, and let her swing in the wind. Not least because I don’t believe you lose 30+ years of anti-EU sentiment overnight, and I reckon Corbyn will privately be quite happy with a hard Brexit, not least because it gives him a stick to beat the Tories with, and there could be a Tory split over it as well, and if he were to get into power not having the EU breathing down his neck would be a plus for a Hard Left Prime Minister. It might well also purge a few Blairites from Labours ranks as well, which would be another bonus as far as Corbyn is concerned.