Thanks for your patience. First of all I should clarify that it is not illegal for an employee to be off sick from one job and receive sick pay, whilst continuing to work for another employer. Many employers incorrectly refer to this as the employee 'defrauding' the company and even take disciplinary action, up to and including dismissal.
However, the legal position is that as long as the employee is genuinely certified unfit to work and this affects their ability to carry out the specific job from which they are signed off, they are able to be off sick from it. In the meantime if they are capable of performing another job, which is not affected by the reasons for being signed off sick, they can do so. For example, someone is signed off sick due to workplace stress as a result of heavy workload or bullying by colleagues. The stress prevents them from working in that specific job as the stressors are directly connected to it. It does not prevent them from continuing to work in another, less stressful job, where no stressors are present.
As to the legal costs you cannot pursue them in the small claims court. Such costs are to be determined by a costs hearing I the venue in which they were incurred, so it would have to be the tribunal which make a decision on whether the claimant would be made liable for any of these costs and you would have to pursue them via that venue through a costs hearing at the end.
I hope this has answered your query. I would be grateful if you could please take a second to leave a positive rating (3, 4 or 5 stars) as that is an important part of our process and recognises the time I have spent assisting you. If you need me to clarify anything before you go - please get back to me on here and I will assist further as best as I can. Thank you