Hi Kaylaw,
Great that you're coming to Kenya
Well hopefully my comments will assist you.
Food: Depends on where you go to shop. Going to a grocery shop is definitely expensive. Going to the market is definitely a cheaper option. Although in lavington you will also find several ladies on some corners selling vegetables... so no worries. You might spend about Ksh. 5,000 per month
Phone credit: Depends on which service provider....plus they are all battling for customers so you'll get good rates. You can opt to take a safaricom postpay line and pay Ksh. 1,000 per month. That's what I use
Fare to Town from Lavington: roughly about Ksh. 30 - 40 depending on the time.
Room Rent: Ksh. 15,000 is definitely a good deal for a serviced apartment in Lavington. Good idea to use public transport. Taxis will dent your pocket.
For your friend's budget holiday package.. you can email:
tours@@#$%&.com. You'll get all the info you need.