Im sorry to hear about your friend’s difficulties at work, it can really make going to work unpleasant. I’d suggest your friend continues to take Union advice and discusses the particulars of the concerns eg specific examples, dates, witnesses etc. They can then gauge the level of concern and hopefully suggest a proportionate means of trying to resolve the issue. Are there any similarly concerned colleagues who would want to raise similar concerns? Your friend would need to be careful scoping that out though, incase word got back to the person complained about. Any unpleasant/unprofessional written communications are best printed and shared too, although most of these issues tend to happen verbally.
I think your friend’s starting point needs to be ‘what outcome am I looking for?’ ‘Is this realistic knowing the colleague concerned?’
Some who have complaints raised against them are mortified/upset and things get sorted quickly. The majority however are defensive and deny the allegations. Your friend will be in the best position, knowing her colleague, to make a guess at how any complaint, informal or formal, will be received. Without evidence, a complaint won’t be upheld. Even if a complaint is upheld, your friend should work on the presumption that they will still be required to work together with them.
If the unprofessional behaviours are relatively subtle with no witnesses, it’s a tricky one. Informal and formal grievances don’t tend to improve relations, unfortunately. It’s not uncommon for those raising complaints to seek a transfer elsewhere after its concluded as they end up feeling they aren’t able to work with the person they complained about anymore, understandably, but employers aren’t obliged to create an alternative job elsewhere to enable that.
If your friend wishes to take action, it’s best to start with the lowest level of intervention that they feel comfortable with. Options might include - informally approaching them privately to discuss directly; informal meeting with management (and union reps for both parties?) to discuss your friend’s perception of colleague’s behaviour towards them, and how this makes them feel at work (your friend is entitled to her perceptions and feelings, without needing to evidence these specifically, so this might be an easier way to phrase the issues where evidence is scant); another ‘level up’, depending on what the employer can arrange, is mediation - the colleague would need to agree to participate with your friend, however. The employer can’t oblige them (although it doesn’t look great for someone in a more senior role to refuse to engage in concerns raised about them, IMO).
I think, realistically, your friend needs to be applying for new positions at the same time. If employed by a local authority, HR/their union may be able to advise on whether there is a list for those putting themselves forward for voluntary redeployment in the event of future restructuring.
It’s not uncommon for individuals to end up on long term sick leave with stress due to such workplace issues, which can make it even trickier to proactively search for a new position, so it’s best to actively pursue that now I think before getting too ground down by it all.
Wishing your friend all the best with whatever they decide to do.