I don't claim legal qualifications either; but we're in the land of Torts here - civil wrongs, for which, as any lawyer will tell you, the remedy lies in an action for damages. The case isn't actually about whether men are allowed in the female changing room - the case is about whether SP suffered discrimination, harassment and victimization, and if so, how much money she deserves to put her right. Each of those things requires, according to the Equality Act 2010, a very definite set of findings from the tribunal which are very fact specific.
For example, under victimization, the act says "Giving false evidence or information, or making a false allegation, is not a protected act if the evidence or information is given, or the allegation is made, in bad faith." So one way for JR to proceed is to show it's more likely than not that the allegation is made "in bad faith". Even if the law says DU shouldn't be in the CR, if the allegation of harrassment was made "in bad faith" NHSF is off the hook on the count of victimization.
For harassment, the act says that in judging the conduct complained of, "...each of the following must be taken into account—
(a)the perception of B; (b)the other circumstances of the case; (c)whether it is reasonable for the conduct to have that effect."
There's a lot of leeway there for JR to argue that the "other circumstances" mean that what occurred doesn't amount to harassment. Say, perhaps because SP always changed in a cubicle, even when DU wasn't present. Even if DU should not have been allowed in the CR. JR gets the big bucks, not me, so that's probably a trite example, but you get the idea.
I guess in a way it's a bit like skittles: JR has a number of different pins that she needs to knock down. She may very well knock quite a few of them over, even if not all.