Happy New Year 2024 (and sorry for the forum being offline some hours) /DM
There are two phases of early to middle Holocene warmth in south-central Iceland in the last 10.2 ka, from <10.2–8.7 ka ago and coincident with peak summer solar insolation at that latitude at 11 ka ago, and peak warmth from 7.35–5.5 ka ago which lags peak insolation by several millennia. Langjökull, Iceland's second largest ice cap, largely disappeared during these two phases indicating temperatures were warmer than today.
It looks like I can read the paper... It may be because I have installed a firefox plugin that is called "unpaywall", it does look for free copy available elsewhere among a list of available sources. http://unpaywall.org/
This paper from lakebed samples shows that iceland was at most 2-3C warmer than the 1961-1990 values (and most likely around 1.5C) https://www.researchgate.net/profile/Peter_Langdon3/publication/223633044_Early_Holocene_climate_variability_and_the_timing_and_extent_of_the_Holocene_Thermal_Maximum_HTM_in_Northern_Iceland/links/02e7e531fe097c774d000000/Early-Holocene-climate-variability-and-the-timing-and-extent-of-the-Holocene-Thermal-Maximum-HTM-in-Northern-Iceland.pdfhowever, it is also known that variations in the AMOC (gulf stream) current in the north atlantic produces large temperature swings. It is these swings that have caused large variability in the Greenland ice core sample (and european temperature record). Lukewarmist will often attach themselves to the intentional lies of paid deniers who present these regional variations as global proxies which they are obviously not. http://www.nature.com/nature/journal/v457/n7230/full/nature07717.htmlFinally, the real issue is that this person you are talking about has a fundamental misconception of what effect the Milankovich cycles have on northern hemisphere temperatures.