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Since she is operated for stomach she is not supposed to take medicines ;in such situation you can do the following:-
--you use a condom.
--you can do sex during safe period that is for 7 days from day of menses(before menses & after menses)
--IUCD Intra uterine contraceptive
device like copper-T.
The first 2 options you can decide yourself ;just in case you need to ask any queries regarding copper-T;please consult a GYNAEC