Internet Connection Suffers when playing Single Player Minecraft?

My friend has a problem when she plays Minecraft Single Player. We always like to talk on Skype and she really enjoys to play Minecraft. But the problem is, when talking on Skype and when she launches Minecraft, her internet connection just goes all to hell. Anything else seems completely fine, but only with Minecraft Single player. Even when just launching Minecraft, it kills her connection. I'm pretty confused. She's using a wireless connection, which it's running pretty decently. She has a dedicated wireless card, too. I'm just a bit puzzled on why this problem is occurring. I'm not sure if it has anything to do with her processor since her processor is pretty weak. Here's her computer specs:

Motherboard: MSI 970A-G46 AM3+
CPU: AMD Athlon II X2 260 @ 3.2GHz
RAM: G.SKILL Ripjaws Series 4GB
PSU: Antec 550 Watt Bronze
GPU: Sapphire Radeon HD 7770 GHz Edition

Any help at all? /

If she has a slow internet connection, using wireless will only make it worse.