From the employer's point of view, they do need to keep neutral right now; they have to make sure any investigation is fair & respects the rights of both parties. Unfortunately some people misinterpret this as not offering any support to the person making the allegations, as your friend has experienced. As rockchick said, she needs to be clear on what she wants the short and long term outcomes to be - does she want to go back to work while this is being sorted out? Does she want to not have to work with him again ever? Is it okay if they are on the same site? Does she want him to be sacked? She may not get exactly what she is looking for, but if she doesn't make it clear then no-one can help her. Other practical things she should do now:
-Get a copy of her company's harassment and/or grievance policy & process. This should be freely available to all staff in a handbook or on a company intranet/other shared area. She should ask her manager (or HR department if there is one) for a copy. Then make sure she has followed the processes outlined in that, eg you say she has made a formal complaint - but did she need to do it on a form? who did she need to send it to? Cross the t's & dot the i's! If they don't have any policies in place, she should get the guidelines from Acas (free to download from their website) and state in writing (email is fine) to her manager that she assumes they are following those.
-They should be instigating a formal investigation. She should ask her manager who will be leading this and when she can expect to meet with them to discuss her allegations and how they will do the investigation. This can be done while she is off sick - I would expect to see a doctor's note or Occupational Health report (depending on the size & practice of the company) stating that she is fit to engage in the process but not to work.
-However it's not a terrible suggestion from them for her to return work while this is going on working at a different site to him in the short term. I would usually expect the person the allegations were made against to be the one who moves rather than her - if she feels able to return to work then she should ask for him to be moved not her. In fact, I would also ask whether they are considering suspension for him; these are very serious allegations, and could be considered gross misconduct. Most companies' policies that I have seen would allow for this. I can't remember off the top of my head but I think Acas cover for it as well - she should check!
-Joining a TU is a great idea, however it's worth bearing in mind that as this is now a "live" issue they may not offer support if she's not been a member previously. However she should be able to take a work colleague with her to formal meetings in the process to offer her support. Who does she trust who is sensible and can help her?
-Finally, make sure she is getting support for herself. If she's signed off sick, what support is she getting from her GP? If it's a decent sized company, they may have employee welfare support (sometimes called employee assistance programmes) - they usually take the form of free, confidential helplines, and some can also offer free face to face counselling as well. She should ask if this is available to her.
Hope this is helpful, and best of luck to your friend - horrible thing to go through.