Depending on aircraft type and weight, flaps down approach speeds for most military fighters is in the range of 120 to 150 KIAS. This is within the speed range of medium to higher performance GA aircraft, but a bit too fast for most fixed pitch prop, fixed landing gear aircraft in common use. Some fighters are quite controllable at high AoA at speeds lower than optimal for approach, but it is unlikely that a fighter pilot would be comfortable flying formation at such a slow speed.
Additionally, there is almost no assistance a military jet could provide to a GA aircraft that would require the two to be in close formation at matching airspeed. The nearest situation necessitating actual close formation flight would be to provide visual verification whether landing gear were fully extended. While close up inspection of an over-center lock is possible to confirm, a general up, down, or stuck halfway assessment is possible from quite a bit further.
Even if it were ideal or possible, unbriefed formation flight between dissimilar aircraft with a GA pilot of unknown proficiency presents its own set of risks. Communication relay to ATC, assigning headings or leading a lost-comm pilot to a hole in the clouds, etc. are legit forms of assistance that could be provided from an interceptor orbiting higher overhead, or leading on at a slightly higher airspeed.