It's important not to get overdramatic about this. Taking each issue at face value, yes your colleague's boss is perfectly right to say that she can only bring a colleague or union rep to her grievance, there's nothing wrong with that.
Suspension isn't disciplinary action. You have not been disciplined and it doesn't sound as though your suspension is pending any disciplinary action against you. Have you actually got any reason to think you've been suspended because you agreed to be this woman's supporting person? You say it's because of a 'cash issue' which presumably is (at least supposedly) unrelated to her grievance. What has this cash issue got to do with you, anything? Are they alleging that you had some part in it? In which case they'd be notifying you of possible disciplinary action against you. Just trying to establish whether it's reasonable to suspend you pending an investigation into it and disciplinary action against someone else.
If what they are alleging against your colleague might possibly amount to theft, it might well be gross misconduct. Could that be the case, could it potentially be theft? If they are bringing disciplinary action against her and have no evidence or anything, then obviously that's not on. Did she pay short? Has it just happened? Again trying to establish whether the fact that they've raised this discplinary issue right now is a reasonable thing for them to do.
Don't get me wrong, there may well be some discrimination involved here, or unfair treatment of you for wanting to accompany someone, but based on what you've said so far, it's by no means certain. I can't see anything that implies constructive dismissal would be a reasonable claim at all.
Your colleague needs to ensure she has all the information about what the allegations are against her and prepare a suitable defence accordingly. She should refuse to answer any claims made in the meeting itself that she wasn't previously made aware of on the basis that she was unable to prepare a suitable defence. She should ensure they follow a reasonable process and can appeal a decision afterwards. If she feels she has adequate reason to believe disciplinary action was completely unwarranted or unreasonable, and possibly related to the fact that she has raised a grievance for sexual harassment, that will of course be relevant.
You should continue to accompany your colleague should you wish to, to her grievance and disciplinary meetings. You should also write to your employer and request further information about your own suspension, including exact clarification about why it is necessary. Depending on the response, you can then decide whether you feel you have reason to believe it is to 'punish' you for accompanying someone and take action accordingly.