Mtal Blitz simulates time trouble, but the real goal of a chess player must be to not entering in time trouble at all!
Slow games give improvement, but in Blitz someone only uses the skils that he had learn before or that someone have. (thats why, it is easy to blame about openings when the real problem is the lack of correct thinking process that someone only learns when playing slow games or analysing positions). It is good to test openings, but Blitz have limitations and the best players only play Blitz for fun and on acasion after a long study session from time to time. It is better for someone beginner to intermediate to know exactly how many games he will play to not get excited and play all day.
Botvinnik once said: "I played a Blitz game once. It was on a train"
Forget about Blitz for now and insted of opening books, it is much better for you and your friend to pick Tal book and start to analyse the games, You can arrange a system of points to see who scored more by guessing (the correct way) the moves and see what moves you failled and why, and correct your thinking. Read the comments, analyse variations that are not on the book, then see what computer says. For a book with 600 pages, that work must take several months of great enjoyment and then if everything was made the correct way and your percentage of correct moves is very high, only then Blitz will be a piece of cake.
Petrosian moves are dificult to find without training and analysis (deep positional and strategic player who uses prophylaxis and limitation of other players moves), it is better to start with Tal book and then Capablanca to follow pure logic. Tartakower "500 master games of chess" is another possible book for you and your friend.