Masturbation is not specifically mentioned in the Bible. Any interpretation that it is a sin must be an application of some other Scripture regarding sexuality. Some of these certainly could imply that masturbation under some circumstances could be sin. For instance, if masturbation is used as a way to deny sex to your spouse, that would be destructive and go against the 1 Corinthians 7 principles Paul describes. If masturbation is accompanied by fantasies of extra-marital relations, it may fit with Jesus' definition of lust in
Matthew 5.
In your situation, which is not unusual because of the physiological differences between men and women, masturbation is not a sin. Often with their faster sexual response, a male has sprinted through the excitation, plateau, ejaculation, and recovery phases while his wife is just beginning to feel turned on.
Manual stimulation may be her only option to achieve orgasm. A thoughtful husband can provide that, giving her not only physical release but a sense of being cherished. That clearly completes the biblical picture of two becoming one.
Masturbation alone can be useful when a couple is not able to be together for sex. This may be the case in many different situations (travel, fatigue, schedule problems, health considerations, even a time of spiritual "fasting" by one mate). I do not see individual masturbation as sinful in these situations as long as the associated thought life honors the marriage vows..cc; lalasticlala mynd44