Your friend is right that more frequent reviews will help you learn and retain material, but changing tasks has a huge transaction cost so I don't think you need to cover every subject every day. Besides the time needed to get oriented on a new agenda item, every transition is an opportunity to get distracted by the lastest facebook/news/email update. So minimize transitions as much as possible. Monolithic scheduling defined by one or two "Most Important Tasks" (aka MITs) is best for avoiding these transaction costs.
To get the benefits of the spacing effect, I highly recommend Anki. I'm a diehard fan of this free application, and I use it to keep me in at least peripheral contact with all my subjects every day. Since reviewing Anki is one 'task' and I've made it a daily ritual, I don't lose tons of time switching activities in order to cover different subjects. If you have an iPhone or iPod, you can get the mobile app for a few bucks and do all your daily reviews during dead time. But even a 30-60 minute Anki ritual on your desktop will pay huge dividends if you stick with it.
Monolithic scheduling has the advantage of clarity (I know this has to get done now, not later), simplicity, and efficiency (minimum task switching time loss).
Monday: MIT = Class A. I must complete all homework for class A today. No other time will be alotted to class A this week, so it's now or never. After finishing assignments and reading, add ~15 Anki cards covering the most important items covered this week for class A. Review all due Anki cards (for all subjects).
Tuesday: MIT = Class B. Do all of B's work, and add new Anki cards for subject B. Review all Anki cards due for all subjects.
Rinse and repeat.
By having each day of the week be defined by one "Most Important Task" plus Anki reps for all subjects, I find that I still get the benefits of the spacing effect without bleeding away huge amounts of time deciding what to do next.
Another advantage of this kind of thinking and planning is that you have semi-firm self-imposed deadlines every day. These hard limits helps you stay on the good side of
Parkinson's Law. If you limit yourself to 4 days on 4 classes, you still have 3 days per week for research or personal projects. But if spend some time every day on each subject, I'd be very surprised if you had any time left over for more interesting activities.