The FCO has warned against all travel to the Syrian border region for years.

If HMG believes the underlying threat level in Turkey has changed significantly, then FCO
advice would be updated. As it hasn’t been, the view is presumably that the ongoing risk is broadly the same as before the recent attack.

OP’s friend shouldn’t worry. If they are of a particularly nervous, or risk averse disposition, then they will need to accept potential financial downside when making travel plans. ‘Events’ will always occur somewhere.