I maintain Zabiha halal but few of my Muslim friends doesn't.
- What to do if he invites me to their home? Can I ask about the meat source?
- If I know if his income is not from a permissible way?
TL;DR: Meat is unlawful until proven lawful, so Muslims need to ensure their meat is halal, but without getting unreasonably suspicious. In the case of reasonable doubts, fatawa offered these alternatives: (a) politely ask the host or someone who knows, (b) suggest fish or vegetable alternatives, (c) offer to buy suitable meat yourself, (d) politely excuse yourself from the event.
(I don't know the answer to 2.; consider asking it separately.)
Ask Imam summarize the situation as follows:
When you are invited to your muslim friends, then use discretion. If you are reasonably certain that the friend is muttaqi (cautious) about his consumption then reside your confidence in him and partake of the food. If you are doubtful about him, then accept the invitation and then politely excuse yourself from attending.
In a fatwa Zabiha-Meat Madness by Abdur-Rahman ibn Yusuf (available at e.g. Zam Zam Academy) gives these possibilities:
When doubtful or unsure about a person’s source of meat, one can take the following steps:
If possible, it is usually best to go directly to the host ... and politely inform him or her ... that you have some strict dietary limitations regarding meat. ...
... let him know ... that you would be perfectly satisfied with alternate options, such as vegetable and fish dishes
If it is difficult to approach the host ... then ask in an appropriate manner someone who may know where the host buys his meat. ...
Volunteer to purchase the meat for the host, and buy it from your trusted source.
The article also writes:
It is necessary that a Muslim do his best to ensure—by asking if necessary but without falling prey to excessive doubt—that the meat he is offered is truly halal, in the correct sense of the word. It is not simply enough to say Bismillah on a piece of meat to make it lawful.
Presumably this is in reference to possible misinterpretations of Sahih al-Bukhari 5507 (sunnah.com), which is used to excuse unreasonable doubt. However, we need to be careful when eating meat, e.g., IslamWeb writes about meat which might be or contain pork:
In principle, all kinds of food are lawful except meat, as it [meat] is in principle forbidden until proven lawful. Therefore, if a Muslim doubts any meat whether or not it is pork (or contains pork), then it is not permissible for him to eat, until he ascertains it is not pork and does not contain pork.
There are several issues here.
❶ - GENERALLY, it is considered disliked to ask a Muslim the source of his income, food, etc and to investigate whether it is Halal or Haram as we ought to have the best view towards each other.
❷ - Specifically, if you know for sure that it is haram, then ofcourse not only do you have to refrain from partaking in such food, but you have an absolute obligation to sincerely advise the person, and to avoid such gatherings due to what it implies. You can be friendly toward them for the purpose of da'wah but not when they are in such clear, flagrant & open violation of the Deen.
❸ - An additional important point to note is that you should be careful who you choose as your friends. Acquaintance and fellow Muslim is different to "Friend". A friend should be chosen based on the highest attributes as you would be upon the way of your friend eventually - either being guided by him or being misled by him, so be aware. In all cases though, you should advise this person to avoid haram.