When a client says their window "mists up on the inside and there's no way to clean it", they almost always mean a failure of the double glazing seal, a problem with no cleaning solution because the mist is literally trapped between the two panes.
Why it happens
The perimeter seal that keeps the gas cavity between the two panes airtight degrades over time (through age, prolonged sun exposure or a manufacturing defect), allowing ambient moisture into the cavity. That moisture condenses on the inner faces of the glass when there is a temperature difference, a mist that stays trapped and can't be cleaned from outside.
Why there's no partial fix
Once the seal has failed, there is no way to "dry out" or clean the inside of the cavity without dismantling the whole unit. The only real solution is to replace the entire double-glazed unit with a new one, not just the visible outer pane.
What to check before quoting the replacement
It's wise to measure the exact dimensions of the current unit (including the total thickness of cavity and panes) so the new unit fits in the same joinery opening without touching the frame, which greatly simplifies the replacement.
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