It's perfectly fine to ask someone about their experience with a company, whether or not they're still working there. People do this all the time and asking the question isn't rude, though the way you phrase it can of course be rude. You'll want to be considerate of the time of your friend and take into account how well you know them and what relationship you have. Sometimes you can send a mail like you did, asking what they thought of the culture, work life balance, technology level or management. But that's requiring an awful lot of work and doesn't give them an easy out, making it harder to say no. Some people may not have the time to answer you or be uncomfortable for a variety of reasons, like your friend in this example. Instead you may want to send an initial mail to ask if they mind answering a few of your questions about that company. You can suggest meeting up, calling them or continuing the conversation via mail. With close friend you can just IM them or call them up spontaneously.
Now, while asking this question is fine, it's also fine for someone to refuse to answer or, more likely, be very guarded with the feedback they give. But the "don't speak ill of a past employer" doesn't really apply in this situation. You wouldn't criticise your employer with acquaintances, colleagues or do it during an interview but it's fine to be honest in a conversation with friends outside work, especially if they asked you for your feedback.