Employers will treat being caught falsely taking sick leave as a disciplinary offence, but otherwise the effect of an employee calling in sick will depend on the organisation’s attendance at work policy, and will generally be minimal, absent significant prior sickness.
Agreed, but I think the OP has suggested they know they won’t get leave as a colleague is away, rather than that they've already had leave declined. In which case it’s strategically probably better not to ask for the day off at all, and simply go sick.
Even if they go sick on a day they’ve had turned down for AL, absent being caught out lying, there’s little an employer can do other than apply their usual attendance policy.
The harsh reality is that recruitment is extremely expensive in terms of budget and manager time, and they have plenty of applicants for these roles, so don’t need to be particularly accommodating - that’s especially the case for new entrants to the industry. If they have a finite number of interview slots, and someone can’t make it/is awkward, they can simply move onto the next person on the list.