That can work fine and serves the same purpose on the trainer as if it was setup on a bike.
What is the actual bike planned for this combo?
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Depending on the bike he has, getting proper chain tension could be an issue. It’s possible to setup a derailleur for good alignment, or even use a single speed tensioner to convert a regular geared bike if that’s the plan.
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Do note that a typical single speed bike is not a good fit for this, due to the typical 120mm axle width that is not support by any DD trainers I have seen. Not to mention getting chain tension with slotted dropouts is also tricky with these trainers.