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Does your friend know when and where her grandfather died?

I think the Thomas William Jackson born in 3rd quarter 1927 may be the same as Thomas William Jackson dob 24 July 1927 who died in St Helens district in 1989.

Maybe a chance his bio father was Ratcliffe?

Cas

My thought too.  There aren't any Ratcliffes (or Radcliffes) in Horncastle that I can see, however there are some people named Rockcliffe who are living there up until the 1881 census, but not after then.

It isn't unusual to find a person recorded twice, if they are staying away from home on census night and someone thinks that they should write down everyone who is normally in a household, even those are elsewhere on that date. It looks as though that is what happened with Jane in 1841.

Jane Walker, daughter of Richard (labourer) and Eliza was baptised at Tong on 2 December 1835.
If this is your Jane, in 1851 she is probably Jane Walker, aged 16 born at Tong employed as a servant in the household of Hannah Howell in Donington.

The use of both Cross and Capstick for Ellen's second marriage would usual suggests divorce from rather than death of the first husband, but here you say Richard Capstick definitely died

Sorry jaywit - a small diversion from your query for anyone who doesn't know this. 
The GRO marriage index, at some times, shows a divorced woman indexed both under her previous married name and her maiden name.
 I don't think that widows are indexed this way at any time, although of course the certificate itself will normally give both names.

The UKBMD indexes https://www.ukbmd.org.uk/
(which don't cover all counties) sometimes show maiden and previous married names for marriages, and also birth registrations, regardless of whether the wife/mother was widowed or divorced.  I don't know what governs whether they are shown or not, but I know  that they aren't always.

There's a Hannah Lowndes baptised 10 June 1804 at Mobberley d/o Samuel and Ann.
Or Hannah Lowndes d/o Martha 7 April 1805 at Mobberley