Given the timeline’s mentioning of Natasha being paid by Bob to walk Chris to the bus stop in middle school I would assume it probably happened early on in high school. With Chris being the basketball team’s water boy beginning freshman year, Bob almost certainly had to have had some involvement in at least wrangling Chris’ retardation at the school early so he wouldn’t end up the punching bag bitch boy like a nerd character in an 80’s high school flick. When parents get involved in school early on, they end up knowing kids quicker and who among them is more compatible with their own kid, or in Chris’ case “most likely to sympathize with retardation”. Regarding your point I always found it funny that Chris never brought up explicit dates when he started being “friends” with these girls simply because he’s the type to hyper focus on that.