他去北京看了朋友，通常是侧重说，他去了北京看了朋友，（现在已经回来了）。 他去北京看朋友了，通常是侧重说，他去北京看朋友了，（现在不在你正在说话的地点，去北京还没有回来）。 当然根据说话语调的侧重，强调的侧重不一样，对应的发问的问题也不同。

The meaning is different to some extent.他去北京看了朋友.It means that "he" went to Beijing,and he visit his friend.But “他去北京看朋友了”.It means that he went to visit his friend,and his friend is in Beijing,so he need to Beijing.Actually, the difference is so small that it wont‘t be emphised in daily conversation.

hope this can help u : 问:他去北京做了什么？ 答:他去北京看了朋友。 问: 他去了哪里？/ 他为什么去北京？ 答: 他去北京看朋友了。 The first one emphasizes that he has visited this friends, second one emphasizes that his purpose to go to Beijing is for visiting his friends.

I would say, it's the same thing.