DD-WRT with OpenVPN is probably the lowest-cost solution, though it takes a little work to set up.

If she's technically inclined (IE, even more work to set up), Sonicwall TZ-100's can be had via Fleabay for $150 or so - I picked up a new one last winter in that price range. The Sonicwall units support L2TP/IPSec (mine works with Android & Apple built-in clients just fine), SSL-VPN, and straight IPSec (with Sonicwall clients). Sonicwall will try and get you to subscribe to their update/security services - you don't need to pay for that to make VPN work.

I've got both DD-WRT/OpenVPN and Sonicwall running to my home network. Both work OK - Sonicwall offers some additional security options that are easier to set up. For example, I can tunnel 100% of my traffic to Sonicwall with a couple of settings in the firewall (and I use it, especially when using an unknown or open network with my laptop) - it takes more work to set that up on OpenVPN.

Either one should allow full access to the local network and any device on the network, but one DOES need to make sure that the internal routing options are properly set on whichever firewall you use.

She'll also need a static IP or DDNS service to make it work. Static IPs generally cost more from the ISP; DDNS usually require some kind of subscription. Personally, I use static IP.