OK, I know anyone interested enough to read this stuff will be well aware that the Big Fight never took place - because that awkward King Richard II stopped it before the first round even started. By doing so he cheesed off almost everyone alive then and every historian since.
Richard was in a dilemma. If Thomas Mowbray had won, how would old Gaunt have reacted? Let's just imagine Henry Bolingbroke, defeated but not killed, going up for the chop. Would Gaunt have sat there and let it happen? Assuming he did, it wouldn't exactly do much for his relationship with the King, would it? Moreover, Mowbray had proved himself a bit of a loose cannon. Lord knows what he would have done - vindicated in this quarrel - for an encore.
What if Bolingbroke had won? Well, that would give a massive boost to an individual who in Richard's books was already far too popular. Plus it would open up the question of Thomas of Woodstock's death/murder/private execution, which Bolingbroke obviously saw as a Bad Thing. Whereas Richard undoubtedly saw it as a Good Thing, and wanted it forgotten. Preferably yesterday.
So Richard stopped the fight and banished them both, Mowbray for life, Bolingbroke for 10 years. In effect he decided they were both guilty, but Bolingbroke not quite as guilty as Mowbray. Lots of people, then and subsequently, thought this harsh on poor old Henry. In terms of justice they were probably right, but then again, medieval justice, especially in the area of high politics, was not noted for being fair and reasonable.
Bolingbroke was told that if his father died during his absence he would be allowed to inherit, and he was even given power to appoint attorneys to act for him in that event. It appears Gaunt was in reasonable health at this time, and his early death was not anticipated.