Replacing only one strut with something very similar

The risks are the replacement used strut will behave differently than the strut already installed and remaining on the vehicle, and may fail differently (earlier, later, and/or in a different manner). A side-to-side inequality may also cause the car to handle differently than it does now, and behave differently in right or left turns.

Unfortunately, there's no way AFAIK to examine and assess these chances. That the proposed used replacement is of a similar vintage (age and use) is better than if the failed strut were replaced with a new one; adding a new one would create a larger inequality between the two struts. OTOH, having side-to-side inequality is a bad thing, particularly because struts are significant parts of the front suspension and steering. Issues there can significantly affect you, your passengers, and other road users.

For myself, using a similar age and mileage replacement strut would carry too high a risk.

Replacing a leaking strut with a used one in better condition is probably fine, it's doubtful there will be any real issues with it, although there's always some risk. You might get a bit of asymmetry, realistically though you're unlikely to notice unless you push the limits of the performance.

To me the main logic to replacing in pairs is that if one side is leaking the other is probably not far behind, so although you can get away with kicking the can down the road by replacing with used, that can is probably not going to roll vary far and you'll be back in a year with the other side leaking. You'll need to spend the money soon, and until you do you know you are driving on 2 worn struts, so I would take the reduced price offer and replace both struts with new ones.