Hey physiosmith,
My experience with athletes and amature athletes is that they usually know what is bothering them as they have had experience in the past with the same injury or they know their anatomy because of their trainers and coaches. As he is only experiencing pain while playing soccer and not running, it is safe to assume that the hip flexor is the issue. Of course there is no way to know for sure unless an assessment is done but I am not going to parrot other advice that has been given here.
To assume the adductor is the primary cause MAY be too premature as the functions of the adductors and abductors are to stabilize the leg, to aim it if you will, for the major movements of soccer. Remember there are cleats at the bottom of those boots (main causes of all kinds of injuries btw!!!!!)so the adductors dont have to provide stability in the same way the skaters do and dont suffer the same forces that they need to compensate for.
The rule to finish a sprint and to follow through on a kick is to prevent explosive decelleration as the muscle eccentrically contracts. To finish a sprint is actually pretty easy to do in soccer as a good athlete will only hold on to the ball for a few seconds. If a defender has an attacker covered the movements of both players will not exceed basic speeds and explosive decelleration will not be necessary. Athletes cant sprint side to side no matter how tallented they are.
I would give the same recommendations to strengthen hipflexors that I give my skaters. First, ensure full dorsiflexion has been achieved. Then ballance lunges and single leg squats that minimize hamstring involvement. As I have said previously, a return to sport asap is important even if the activities have been limited according to the current state of the condition. Also, jogging is not a good indicator of a complete recovery. In fact, it goves the athlete confidence to hurt themselves again.
I hope this clears the air a little.
Adamo