I got to step in here as a graduate of a liberal arts school (and current Ph.D. student at a major research university). I got news for everyone, the overwhelming majority of Ph.D. students in math and physical sciences come from liberal arts school. The reason your friend may want to go to a liberal arts schools is because he prefers to learn math from a professor in classes of <30 rather than from a TA who is usually a grad student teaching only to be paid.
Research experiences at a liberal arts college are for undergraduates while places like big research universities, Ivies, etc. could give a crap about undegrads as the graduate students are the focus because they are RESEARCH universities.