What kind of private information would a bitcoin "middleman" ask from clients? I don't know much about bitcoin so forgive me for the ignorant title and terminology. My mom recently got into bitcoin and told me she was investing her money in it or something like that and she's been doing these transactions through a company (whom I referred to as "middle men" in the title). These middle men recently asked her to provide pictures of her passport with her in it hold it up and also a pic of her credit card bill with her in it as well ; apparently to verify that she's a resident where we live (in Hong Kong, China). 

I thought it was a bit suspicious that they asked for such private information. Is this a normal practice of bitcoin middle men/compamies?
When dealing with established companies as a middelman (usually an exchange), it is common to go through a form of know your customer (KYC). This usually involved some form of national ID, and a bank statement or other proof of residence/address, and is usually done to comply with local laws regarding movement and sale of currencies.

You should, however, be extremely wary of dealing with random companies or people. Make sure you are dealing with a reputable exchange (check review on reddir, yours.org, steem, and other such platforms), and as far as possible, keep any BTC or other crypto in a wallet where you control the private keys (such as electrum, Ledger, Trezor etc). If someone offers to do mining on your behalf, it is almost certainly a scam. If someone says you need to pay to unlock the BTC, it is a scam.

Verify you are dealing with a legitimate entity.