Because the verb "iść" (to go) means not only "iść na piechotę" (to walk), but also "mieć zamiar/zamierzać/udać się/jechać/wybrać się/uczęszczać" (to go/to be going to/to intend/to attend), the translation "If she goes, he goes", just like the other one, "If she is going, he is going", perfectly matches the Polish sentence "Jeśli ona idzie, on idzie".
If you speak about the woman, and her shadow/stalker walking with her
at the same time you say the sentence, you would mean:
Jeśli ona idzie, on idzie - If she is walking, it/he is walking
If you speak at some other time, you have to use a different Polish verb:
Jeśli ona chodzi, on chodzi - If she walks, it/he walks
I'm not saying otherwise, but that's "on pójdzie", not "on idzie". We strictly keep to the equivalence of tenses, because without that, the learners may quickly become confused as to what is Present and what is Future in Polish. Even if the meaning is virtually the same.
After all, if you know some Polish but you see "pójdzie" for the first time, it doesn't really look like Future Tense. Future Simple verbs look just like Present Tense verbs, it's just that they are perfective instead of imperfective. Meanwhile, in English, the grammatical structure (will go vs goes/is going) makes everything obvious even if you don't know the particular verb.