He with his friend is going to 为什么用is

with 是伴随状态，主语只是He，这句话也可以写成He is going to XX with his friend.

用with的话是就远原则，动词由he来决定
（百科）代表词汇：as well as；（together/along/combined）with；rather than；except；besides；but；including；in addition to；apart from, like, as much as, no less than,

with his friend 是插入语，主语是he 谓语用单数is
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有with的时候谓语是根据with后面的词来用

因为 在 he with his friend 的结构中 主语是 he类似用法如：together with,as well as【亮点来喽】就远原则：谓语动词与前面主语一致 代表词汇：as well as；（together/along/combined）with；rather than；except...