What he's charged with and what, if anything, he's convicted of matter. There is a huge difference between vehicular manslaughter while intoxicated and vehicular manslaughter while intoxicated with gross negligence. Vehicular manslaughter while intoxicated carries 16 months, 2 years or 4 years in state prison. Vehicular manslaughter while intoxicated and with gross negligence carries 4, 6 or 10 years.
There are also theories under which he could be charged with second degree murder, landing him 15 to life.
If he is sentenced under one of the manslaughter theories, the first question is going to be whether he gets probation or is sent to state prison. It's very likely that he will go to prison, but that depends on the facts of the case. If sent to prison, the judge should start at the mid term unless factors in aggravation or mitigation push him/her toward the lower or upper sentence.
There's just no "average" sentence, as every case is unique. No more so than in cases where there's the loss of life.
Your friend needs a very good attorney in their corner to either fight against the charges and/or work out the very best sentence possible.
There is no simple answer to this question. Nearly every DUI manslaughter case is unique in some way, shape or form. Sentences can vary anywhere from as little as less than a year to as much as 15 plus years depending on the facts. You should contact a local DUI attorney who handles such types of matters as not all DUI attorneys handle vehicular manslaughter. Always ask the attorneys who you may be interviewing as to whether they have any actual experience in this area of DUI law as you need someone who knows what they are doing as the penalties are so great. I wish I could provide you with more information, but in light of the brevity of your post, it is a more difficult question to answer.
I agree with both of my colleagues in this particular case. The "average jail sentence" is a number based on a wide variety of sentences in cases both worse and better than your friend's. Therefore, the answer probably doesn't have any real meaning to anyone. The likely or possible outcomes in your friends case depend almost entirely on the facts that are either proven at trial or agreed upon between the defense and the DA.
Because of the loss of life, and the very real possibility of state prison, your friend should retain someone who is very experienced with both DUI cases and the science behind them, as well as more serious felony matters. Your friend should also expect that he will probably need the services of a blood alcohol forensic expert and possibly an accident reconstruction expert. It is also sometimes possible for the defense attorney to talk to the insurer and raise funds to hire experts if there is a claim against the driver's insurance as a result of the accident.