Immune boosting supplement for older vulnerable person

Hello! I'm looking for recommendations for an immune boosting supplement for my older mum. She has complex needs following a stroke earlier this year, and is therefore vulnerable to infections (e.g UTIs, pneumonia). The supplement will need to be administered through a PEG feed into her stomach, so it'll need to be dissolvable tablets ideally as liquids can be too sticky to go through tube. Also, the supplement needs to be gentle on her tummy, so it doesn't cause loose bowel movements or other unwelcome side effects. Any recommendations in the UK are most welcome.