There are several ways you could risk discrimination claims. If you are inconsistent in treating employees and this inconsistency can be linked to a protected characteristic, the dismissed employee could bring a discrimination claim.
Where the dismissal is for sickness and capability, you have additional duties to support your employee if the illness is a ‘disability’ under the Equality Act including making reasonable adjustments. The legal definition of disability can be quite different from the common understanding of the word. You should seek advice to understand these responsibilities before looking to dismiss for sickness or sickness absence.