I would recommend reading basics of trading first.
Exchange platforms I am using without a problem and can recommend are:
1.Binance.com
2.Bitfinex.com
3.Poloniex.com
3.Kucoin.com
If you want to buy Crypro with Fiat I suggest:
1.Cex.io
2.Kraken
For tracking prices, the best site is coinmarketcap.com
Good mobile apps for portfolio management: Delta and blockfolio.
Are these mobile apps for iOS and Android devices?
Those are exchange platforms and not mobile apps. You will need to be registered on any of them first to be able to trade cryptocurrency and I hope your friends know how to trade, because these are some of the things that are online that I expect anyone to have been at least familiar with before going ahead to start trading cryptocurrency in the first place as it entails a lot.
After registering on an exchange, I would consider using applications like tab trader if you are looking for a good mobile app and then make use of your API to trade on any of the exchanges like bittrex or binance. I really do not always fancy using the mobile app of the exchanges directly anyway for the purpose of doing analysis.