If you have serious concerns about the qualifications of a job candidate who has asked you for a written recommendation, you should:

elect not to write the recommendation.

Explanation:

A job recommendation is a way of vouching for a person's behaviour, honesty, and qualifications to perform a job.

On important step before giving recommendations is to think carefully if you know and trust the person sufficiently to vouch for him.

If a recommendation is written about person and he fails to perform well, the person that wrote the recommendation will lose credibility and his career may be adversely affected.

In this instance since there are serious concerns about the qualifications of a job candidate who has asked you for a written recommendation, elect not to write the recommendation